Carburetors

Vocabulary Fuel atomization 9. Low speed circuit Carburetor 10. Main circuit Venturi 11.Power enrichment Vacuum 12. Acceleration circuit Air/fuel ratio 13. Choke circuit Hydro carbon Float circuit Idle circuit

Carburetors mix fuel with air in the correct ratio for all engine operating conditions. Early Carburetors were called a mixing valve

Air – Fuel ratio chart

Air – Fuel ratio

Carburetor barrel / Venturi orientation

Booster Venturi Increases Vacuum in Venturi area

A Basic Carburetor Venturi - Creates vacuum Throttle - ( Controls air & vacuum) Fuel flow s Because of vacuum Fuel Under atmospheric pressure

--- CFM vent

Any pressure lower than atmospheric is vacuum. Types of vacuum

The carburetor must provide the engine with the correct mixture for all driving conditions. This is very difficult to accomplish. Different carburetor “circuits or fuel pathways are used to achieve smooth operation.

7 Carburetor circuits 1. Float circuit 2. Idle circuit 3. low speed circuit 4. Main circuit 5. power circuit 6. acceleration circuit 7. Choke circuit

Float Circuit The float circuit provides a constant reservoir of fuel for all the carburetor circuits.

Idle circuit The Idle circuit provides a small amount of fuel while the throttle is mostly closed. The point of highest vacuum is below the throttle plate. Mixture screw

Low speed circuit As the throttle is opened just past the idle circuit the low speed port is exposed to vacuum. The low speed circuit helps the carburetor to transition to the main circuit.

Low speed or transfer slot Idle port Transfer slot

Main Circuit The Main circuit functions after the throttle opens about 25% or more. Fuel flows from the float bowl through a jet, up the main nozzle & is discharged in the venturi. Jet

Venturi Jet

Power circuit More fuel is needed under heavy throttle applications so a vacuum operated metering rod is used to provide more fuel through the main circuit. piston metering rod Intake manifold Vacuum line

Acceleration Circuit As the throttle is suddenly opened the air – fuel ratio leans out. This will cause hesitation , back fire & stalling. A small fuel pump squirts extra fuel in to the venturi area.

Accelerator Pump circuit Discharge nozzle Discharge nozzle

Choke Circuit When the choke valve closes & covers the top of the carburetor barrel a high vacuum is generated below the choke plate. This draws a lot of fuel through the main circuit and creates a rich mixture.

Choke valve This rich mixture is only needed when the engine is cold. This is because cold fuel does not atomize well and tends to pool on the walls and floor of the intake manifold. A rich air – fuel ratio of about 10 to 1 is needed so fuel will actually get to the combustion chamber to help the car start.

Choke valve operation Thermostatic coil spring

Choke valve The # one cause of carburetor problems is the choke system. A choke that is improperly adjusted or inoperative can cause hard starting, flooding, poor or no idle, high fuel consumption and a lot of black exhaust out the tail pipe.

2 & 4 barrel carburetors

Carburetors can be rebuilt. But there are many parts and adjustments.

Carburetor problems: The carburetor can not compensate for changes in altitude or air temperature. The carburetor does not like cold air temperatures ( below 32 degrees ) The carb can not compensate for small changes in load during Idle. The carb can not easily meet emission control requirements because is simple is not accurate enough.

Good luck on your unit test!!! See Mr. Agan Before or after school for help on this topic.